Conventional draftsman's ink writing instruments are structured as shown in FIG. 1. In such conventional draftsman's ink writing instruments, an outer case 1 has three cylindrical stages of different external diameters and an axial through bore which likewise has three stages having different internal diameters. The largest diameter portion of the outer case 1 is referred to herein as the ink reservoir 2, and the part of the axial through bore contained therein functions as an ink reservoir. The medium diameter portion of the outer case 1 contains the medium diameter portion of the axial through bore and is referred to herein as an operation part 3. The small diameter portion of the outer case 1 contains the small diameter portion of the axial through bore and is referred to herein as the writing part 4. The writing part 4 contains an ink flow hole 7 which is open to the bottom of the outer case 1, while the ink reservoir part 2 is open at the top of the outer case 1. A tapered annular shoulder 5 is formed in the inner surface of the axial through bore between the reservoir and the portion of the axial through bore in the operation part 3. An external annular step shoulder 6 is formed on the outer case 1 at the level of the bottom of the tapered annular shoulder 5. Further, a tapered annular shoulder 8 is formed in the inner surface of the axial through bore at the juncture between the medium diameter portion and the small diameter portion thereof.
An ink flow stopping piece 9 is inserted in the outer case 1. The ink flow stopping piece 9 has a needle part 10 which is located in the ink flow hole 7 and the end of which projects through the ink flow hole 7. The ink flow stopping piece 9 also has a flowing down prevention part 12 which has a tapered flow stopping surface 11. The tapered flow stopping surface 11 valves the tapered annular shoulder 8 to prevent the flowing down of the ink. The flowing down prevention part 12 is of cylindrical shape and is sized so that an annular gap is present around it. A weight 14 is connected to the upper surface of the flowing down prevention part 12 by a connecting rod 13.
Such a conventional draftsman ink writing instrument prevents external leakage of ink by surface tension when the ink reservoir is filled with ink. The flowing down of the ink is prevented by the annular engagement of the tapered annular shoulder 8 with the corresponding tapered flow stopping surface 11 on the ink flow stopping piece 9, and the tapered flow stopping surface 11 is biased into engagement with the tapered annular shoulder 8 by the force of gravity operating on the weight 14.
When performing character or line writing with the conventional draftsman's instrument shown in FIG. 1, a grip holding member 18 is pivotally supported at the desired angle to the end of a fork-shaped holding member 16 attached to the end of a pen holder 15 by a pin 17 as shown in FIG. 2. The grip holding member 18 grippingly holds the outer surface of the operation part 3 of the outer case 1 with a forked grip holding part 19 attached to the end of the grip holding member 18. The grip holding member 18 keeps the ink flow stopping piece 9 from dropping out by holding the head of the weight 14 with the suspender 20 formed integrally with the forked grip holding part 19 and extending upwardly.
When it is desired to draw a circle, a leg 22 of a compass 21 holds the outer case 1 as shown in FIG. 3. An attachment 24 having a bore 23 is mounted on the leg 22 of the compass 21, and the operation part 3 of the outer case 1 is inserted into the attachment 24 and is gripped by tightening an adjusting screw 25.
When a draftsman performs character or line writing using the pen holder 15 or the compass 21, the needle part 10 is touched to the paper, forcing the ink flow stopping piece 9 upwardly, and the tapered flow stopping surface 11 separates from the tapered annular shoulder 8 to allow ink to flow down. The ink flows down the ink flow hole 7, and a line of the thickness corresponding to the outer diameter of the writing part 4 is formed.
In the above draftsman ink writing instrument, the needle part 10 required to write a fine line is itself very fine. The needle part 10 may be easily bent, particularly when the ink flow stopping piece 9 falls out of the outer case 1. Therefore, a U-shaped clip 26 has been conventionally fitted to the outer wall of the ink reservoir 2 so that the ink flow stopping piece 9 will not fall out when the apparatus is turned upside down.
However, a serious drawback of this conventional construction is that, because one side of the U-shaped clip 26 is placed in the ink reservoir, the ink may leak out due to capillary action.
Moreover, in those apparatuses intended to write thick lines, the needle part 10 is thick, and the total weight of the ink flow stopping piece 9 is large. In that case, the U-shaped clip 26 cannot stop the inadvertent coming out of the ink flow stopping piece 9. Therefore, the ink flow stopping piece 9 comes out when the apparatus is turned upside down, and the ink loses its surface tension and flows out of the ink reservoir. Such an eventuality may cause trouble by making stains on the paper surface or the draftsman's clothes.